The present invention pertains to organic light active devices. More particularly, the present invention pertains to devices and methods for fabricating organic light active devices that can be used for applications such as general lighting, display backlighting, video displays, Internet appliances, electronic books, maps, digital newspapers, stereoscopic vision aides, head mounted displays, advanced vehicle windshields, solar cells, cameras and photodetectors. A multi-color single layer light active device is disclosed. Also disclosed is a sequential burst driving scheme for a multi-color single layer display. Further disclosed are methods for making organic light active material particulate, as well as an organic light active fiber.
A polymer is made up of organic molecules bonded together. For a polymer to be electrically conductive it must act like a metal with the electrons in the bonds mobile and not bound to the atoms making up the organic molecules. A conductive polymer must have alternate single and double bonds, termed conjugated double bonds. Polyacetylene is a simple conjugated polymer. It is made by the polymerization of acetylene. In the early 1970's, a researcher named Shirakawa was studying the polymerization of acetylene. When too much catalyst was added, the mixture seemed to have a metallic appearance. But unlike metals the resulting polyacetylene film was not an electrical conductor. In the mid-1970's the film was reacted with iodine vapor. The result was an extreme increase in the conductivity of the polymer film, and ultimately resulted in a Nobel Prize in Chemistry for the researchers who discovered it.
Although polyacetylene can be made as conductive as some metals, its conductivity drops rapidly in contact with air. This has led to the development of more stable, conjugated polymers, for example, polypyrrol, polyaniline and polytiophene.
There is now intensive development working with conjugated polymers in their un-doped, semiconductive state. It was found that some conjugated polymers exhibit electroluminescence when a voltage is applied. Further, the absorption of light by the semiconductive polymer results in positive and negative charges that produce an electric current. Thus, conjugated polymers can be used to make solar cells and light detectors.
Organic light active material (“OLAM™”) makes use of the relatively recent discovery that polymers can be made to be conductors. Organic light emitting diodes (“OLED”) convert electrical energy into light, behaving as a forward biased pn junction. OLAMs can be light emitters or light detectors, depending on the material composition and the device structure. For the purpose of this disclosure, the term OLAM and OLED can be interchanged. In its basic form, an OLED is comprised of a layer of hole transport material upon which is formed a layer of electron transport material. The interface between these layers forms a heterojunction. These layers are disposed between two electrodes, with the hole transport layer being adjacent to an anode electrode and the electron transport layer being adjacent to a cathode electrode. Upon application of a voltage to the electrodes, electrons and holes are injected from the cathode electrode and the anode electrode. The electron and hole carriers recombine at the heterojunction forming excitons and emitting light.
The basic structure of an OLED display is similar to a conventional LCD, where the reactive material (in the LCD case, a liquid crystal, in the OLED case, a conjugated polymer) is sandwiched between electrodes. When an electric field is applied by the electrodes, the OLED material is brought into an excited energy state, this energy state drops down by the emission of photons, packets of light. Thus, each pixel of the OLED display can be controlled to emit light as needed to create a displayed image.
Besides attractive picture quality, an OLED display device consumes less power than liquid crystal display technologies because it emits its own light and does not need backlighting. OLED displays are thin, lightweight, and may be able to be manufactured on flexible materials such as plastic.
Unlike liquid-crystal displays, OLEDs emit light that can be viewed from any angle, similar to a television screen. As compared to LCDs, OLEDs are expected to be much less expensive to manufacture, use less power to operate, emit brighter and sharper images, and “switch” images faster, meaning that videos or animation run more smoothly.
Recently, an effort has been made to create equipment and provide services for manufacturing OLED screens. The potential OLED display market includes a wide range of electronic products such as mobile phones, personal digital assistants, digital cameras, camcorders, micro-displays, personal computers, Internet appliances and other consumer products.
There is still a need, for example, for a thin, lightweight, flexible, bright, wireless display. Such a device would be self-powered, robust, include a built-in user-input mechanism, and ideally functional as a multipurpose display device for Internet, entertainment, computer, and communication use. The discovery of the OLED phenomenon puts this goal within sight.
However, there are still some technical hurdles that remain to be solved before OLED displays will realize their commercial potential. OLED's light emitting materials do not have a long service life. Presently, optimum performance in commercially viable volume production is achievable only for small screens, around 3.5 inches square or less. Storage lifetimes of at least 5 years are typically required by most consumer and business products, and operating lifetimes of >20,000 hours are relevant for most applications.
Organic-light-emitting-diode technology offers the prospect of flexible displays on plastic substrates and roll-to-roll manufacturing processes. One of the biggest challenges to the OLED display industry is from contamination by water and oxygen. The materials involved in small molecule and polymer OLEDs are vulnerable to contamination by oxygen and water vapor, which can trigger early failure. U.S. Pat. No. 5,247,190 issued to Friend et al., teaches an electroluminescent device comprising a semiconductor layer in the form of a thin dense polymer film comprising at least one conjugated polymer sandwiched between two contact layers that inject holes and electrons into the thin polymer film. The injected holes and electrons result in the emission of light from the thin polymer film.
Recently, there has been activity in developing thin, flexible displays that utilize pixels of electro-luminescent materials, such as OLEDs. Such displays do not require any back lighting since each pixel element generates its own light. Typically, the organic materials are deposited by solution processing such as spin-coating, by vacuum deposition or evaporation. As examples, U.S. Pat. No. 6,395,328, issued to May, teaches an organic light emitting color display wherein a multi-color device is formed by depositing and patterning thin layers of light emissive material. U.S. Pat. No. 5,965,979, issued to Friend, et al., teaches a method of making a light emitting device by laminating two self-supporting components, at least one of which has a thin layer of light emitting layer. U.S. Pat. No. 6,087,196, issued to Strum, et al., teaches a fabrication method for forming organic semiconductor devices using ink jet printing for forming thin layers of organic light emitting material. U.S. Pat. No. 6,416,885 B1, issued to Towns et al., teaches an electro-luminescent device wherein a conductive polymer thin layer is disposed between an organic light emitting thin layer and a charge-injecting thin layer that resists lateral spreading of charge carriers to improve the display characteristics. U.S. Pat. No. 6,48,200 B1, issued to Yamazaki et al., teaches a method of manufacturing an electro-optical device using a relief printing or screen printing method for printing thin layers of electro-optical material. U.S. Pat. No. 6,402,579 B1, issued to Pichler et al., teaches an organic light-emitting device in which a multi-layer structure is formed by DC magnetron sputtering to form multiple thin layers of organic light emitting material. U.S. Pat. No. 6,50,687 B1, issued to Jacobson, teaches an electronically addressable micro-encapsulated ink and display. In accordance with the teachings of this reference, microcapsules are formed with a reflective side and a light absorbing side. The microcapsules act as pixels that can be flipped between the two states, and then keep that state without any additional power. In accordance with the teaching of this reference, a reflective display is produced where the pixels reflect or absorb ambient light depending on the orientation of the microcapsules. U.S. Pat. No. 5,858,561, issued to Epstein et al., teaches a light emitting bipolar device consisting of a thin layer of organic light emitting material sandwiched two layers of insulating material. The device can be operated with AC voltage or DC voltage. U.S. Pat. No. 6,433,355 B1, issued to Riess et al., teaches an organic light emitting device wherein a thin organic film region is disposed between an anode electrode and a cathode electrode, at least one of the electrodes comprises a non-degenerate wide band-gap semiconductor to improve the operating characteristic of the light emitting device. U.S. Pat. No. 6,445,126 B1, issued to Arai et al., teaches an organic light emitting device wherein an organic thin layer is disposed between electrodes. An inorganic electrode or hole injecting thin film is provided to improve efficiency, extend effective life and lower the cost of the light emitting device.
It is known to form a thin OLED layer by various method including vacuum deposition, evaporation or spin coating. Thin layers of hole transport material and then electron transport material are formed by these known methods over a grid of anode electrodes. The anode electrodes are formed on a glass plate. A grid of cathode electrodes is then placed adjacent to the electron transport material supported by a second glass plate. Thus, the basic OLED organic stack is sandwiched between electrodes and glass plate substrates. It is generally very difficult to form the electrodes with the precise alignment needed for forming a pixilated display. This task is made even more difficult in a multicolor display, where the OLED pixels emitting, for example, red, green and blue, are formed side-by-side to fabricate a full color display. Because the OLED material and electrodes can be made transparent, it is possible to stack the color OLED pixels on top of each other, allowing for a higher pixel packing density and thus the potential for a higher resolution display. However, the electrode alignment issue still poses a problem. Typically, the well-known use of shadow masks are employed to fabricate the pixel components. Aligning the shadow masks is difficult, and requires extreme precision.
Currently, inkjet printing has gained ground as a promising fabrication method for making OLED displays. The core of this technology is very mature, and can be found in millions of computer printers around the world. However, there are some serious disadvantages to the adapting of inkjet printing to OLED display fabrication. It is still difficult to lay down precise layers of material using the spray heads of inkjet printers. Inkjet printing does not adequately overcome the problem of material degradation by oxygen and water vapor. Elaborate and expensive materials and fabrication processes are needed to provide adequate encapsulation of the display elements to prevent early degradation of the OLED material due to water and oxygen ingress. As an attempt to solve this contamination problem, Vitex Systems, Sunnyvale, Calif., has developed a barrier material in which a monomer vapor is deposited on a polymer substrate, and then the monomer is polymerized. A thin layer of aluminum oxide a few hundred angstroms thick is deposited on the polymerized surface. This process is repeated a number of times to form an encapsulation barrier over an OLED display. This elaborate encapsulation barrier is an example of the effort taken to prevent water and oxygen from contaminating the easily degraded OLED films that form a conventional OLED display device. Even with this elaborate encapsulation process, the edges of the OLED display still need to be sealed.
It is difficult to align display pixel-sized electrodes and inkjet printed OLED material with the accuracy needed to effect a high resolution display. Due to the very thin layers of material involved in forming conventional OLED devices, shorts and the destruction of pixels result from the inclusion of miniscule foreign particles, requiring the use of expensive clean room or vacuum manufacturing facility for the various prior art fabrication methods. Accordingly there is a need for an improved fabrication method for forming OLED devices.
There is also a need for a multi-color OLED structure whereby two or more colors of light can be produced from a single pixel or OLED device. U.S. Pat. No. 6,117,567, issued to Andersson et al, teaches a light emitting polymer device for obtaining voltage controlled colors based on a thin polymer film incorporating more than one electroluminescent conjugated polymer. The polymer thin film is sandwiched between two electrodes. Upon application of different voltages to the electrodes, different colors of light are emitted from the conjugated polymers contained in the thin film.
Edwin Land introduced a theory of color vision based on center/surround retinex (see, An Alternative Technique for the Computation of the Designator in the Retinex Theory of Color Vision,” Proceedings of the National Academy of Science, Volume 83, pp. 3078-3080, 1986). Land disclosed his retinex theory in “Color Vision and The Natural Image,” Proceedings of the National Academy of Science, Volume 45, pp. 115-129, 1959. These retinex concepts are models for human color perception. Others have shown that a digital image can be improved utilizing the phenomenon of retinex (see, U.S. Pat. No. 5,991,456 issued to Rahman et al, the disclosure of which is incorporated by reference herein). The inventors of the U.S. Pat. No. 5,991,456 patent used Land's retinex theory and devised a method of improving a digital image where the image is initially represented by digital data indexed to represent positions on a display. According to the inventors of the U.S. Pat. No. 5,991,456 patent, an improved digital image can then be displayed based on the adjusted intensity value for each i-th spectral band so-filtered for each position. For color images, a novel color restoration step is added to give the image true-to-life color that closely matches human observation.
Nanoparticles are used in unrelated applications, such as drug deliver devices. Others have shown that very small polymer-based particles can be made by a variety of methods. These nanoparticles vary in size from 10 to 1000 nm. A drug can be dissolved, entrapped, encapsulated or attached to a nanoparticle matrix. Depending on the method of preparation, nanparticles, nanospheres or nanocapsules can be obtianed. (see, Biodegradable Polymeric Nanoparticles as Drug Delivery Devices, K. S., Soppimath et al., Journal of Controlled Release, 70(2001) 1-20).
The current state of the OLED fabrication technology requires the formation of very thin films of organic light emitting material. These thin films are formed by known techniques such as vacuum deposition, screen printing, transfer printing and spin coating, or by the re-purposing of existing technology such as ink jet printing. In any case, the current state of the art has at its core the formation of very thin film layers of organic material. These thin films must also be deposited very uniformly and precisely, which has proven extremely difficult to do. These thin layers of organic material are susceptible to major problems, such as delamination, particularly when applied to a flexible substrate. The extreme thinness of the layers of organic material between conductors also results in electrical shorts easily forming due to even very small specks of dust or other contaminants. Because of this limitation, costly cleanroom facilities must be built and maintained using the conventional OLED thin film fabrication techniques. Organic light emitting devices offer tremendous potential due to the nature of the raw materials, however, the current state of the art fabrication methods are limiting the delivery of this potential to the consumer.